1. Field of the Invention
The invention relates generally to rotating electrical machines, such as motors and generators, and more particularly to direct drive generators with axial air gaps.
2. Description of Related Art
In a typical electrical generator, a rotor with an array of magnetic elements, such as permanent magnets, is driven by a mechanical force to rotate in close proximity to a stationary stator, which carries metallic windings. An air gap is maintained between the magnetic elements of the rotor and the windings of the stator. Typically, the mechanical force used to turn the rotor is supplied by a rotating input shaft, and a gearbox is coupled between the input shaft and the shaft on which the rotor is mounted in order to increase the angular velocity of the rotor shaft as compared with the input shaft.
Electrical generators are used in a variety of contexts, including wind turbines. In a wind turbine, wind provides the mechanical force to drive an electrical generator. In a typical case, the wind drives a set of blades, which cause a hub to rotate an axle. The axle acts as the input shaft for the generator. The generator is generally coupled to the axle through a gearbox.
More recently, so-called “direct drive” generators have shown promise in use with wind turbines. In a direct drive generator, there is no gearbox; the generator rotor turns or rotates at the same rate as the blades and hub. Because a direct drive generator does not use a gearbox, it is typically much larger than a traditional generator, so that it can generate adequate electricity at the slower speed of rotation. Direct drive generator diameters of greater than 3 meters are common in wind turbine applications with a power rating above 2 Megawatts (MW).
In a typical direct drive generator, such as that disclosed in U.S. Pat. No. 7,456,534 to Engstrom, the air gap between the rotor and the stator is a radial air gap—i.e., the gap between the rotor and the stator lies along the radial edge of the rotor, and the stator faces the radial edge of the rotor on the other side of the air gap. In such direct drive generators, one of the main problems is maintaining a uniform air gap. The dimensions of the typical direct drive generator make that difficult—the diameter of the rotor may be 2 meters or more, but the air gap is usually on the order of 5 to 10 millimeters.